radionuclide species at these disposal facilities is of signifi-The adsorption of aqueous Pb(II), EDTA, and Pb(II)-EDTA cant environmental concern because of metal-organic comcomplexes onto TiO 2 were studied at both stoichiometric and nonplex formation and resulting increased mobility of toxic spestoichiometric Pb(II)/EDTA concentrations. For Pb(II)-TiO 2 cies to adjacent groundwater and soil systems (5, 12,18).
and EDTA-TiO 2 , a typical cationic and anionic-type of adsorption
Treatment of waters containing complexed metal species was noted, respectively. For 10 03 and 10 04 M Pb(II)-EDTA sysgenerally requires destruction of the complex. Titanium ditems, near-equal adsorption of Pb(II) and EDTA indicated that oxide (TiO 2 )-assisted photocatalysis, extensively studied for the complex adsorbs as a single species. Also, a ligand-type Pb(II)the oxidation of aqueous organic contaminants, has been EDTA adsorption, i.e., decreasing adsorption with an increase in the pH, was noted. Systems with EDTA greater than Pb(II) recently evaluated for metal-organic complex degradation showed near-zero lead removal; competitive adsorption of EDTA (6,10,15,16). Applications of this technology include 1) and Pb(II)-EDTA onto TiO 2 was suggested to cause this effect. decontamination of groundwater containing metal com-For Pb(II) concentrations (5 1 10 04 and 10 03 M) higher than plexes such as Pb(II)-EDTA, 2) treatment of effluent from EDTA (10 04 M), significantly higher EDTA adsorption at high soil-washing processes that employ chelates such as EDTA pH as compared to individual 10 04 M EDTA and 10 04 M Pb(II)to strip toxic metals from the soil (17), and, 3) treatment EDTA systems was noted. Adsorption modeling was completed of industrial wastewater from plating baths containing metalemploying the geochemical speciation model MINTEQA2 em-
EDTA complexes.
ploying the diffuse layer model. Inner-sphere complexation was As the metal-organic complex is destroyed during phoconsidered to occur between Pb(II), EDTA, Pb(II)-EDTA, and tocatalysis, the organic chelate will be oxidized to various the TiO 2 surface sites. Surface complexes used in the modeling included Ti-O-Pb / , Ti-EDTAH 20 , Ti-EDTA-Pb 0 , and Ti-O-Pb-intermediate products and eventually to CO 2 . The oxida-EDTA 30 . The cationic-type complexation, Ti-O-Pb-EDTA 30 , was tion process occurs either at the TiO 2 surface or a small postulated to explain and model the anomalous EDTA adsorption distance from the surface in the solution phase ( 27 ) . Conas noted for Pb(II) ΓΊ EDTA studies. Results from the present currently, the resulting free metal species can be concenstudy show that the adsorption behavior in aqueous metal/EDTA trated onto TiO 2 particles through either adsorption or systems will change with any variation in the contaminant concenprecipitation reactions. Limited information on EDTA and tration ratios. α§ 1998 Academic Press Pb ( II ) -EDTA adsorption onto TiO 2 is available; however,